Phase transitions in chiral magnets and spin fluctuations

Temperature dependence of the specific heat in MnSi consists of a combination of a sharp first-order peak accompanied by a broad hump. To model such a behavior we study the extended Heisenberg model with competing exchange J and anisotropic Dzyaloshinskii-Moriya D interactions. To take into account the fluctuating nature of the spin density in itinerant magnets we allow spin length fluctuation in our model. Utilizing classical Monte Carlo simulations we find an evolution of the temperature dependence of the specific heat and magnetic susceptibility in a broad range of ratio D/J and their dependence on the amplitude of the allowed spin length fluctuations. While the spin length fluctuations do not change the nature of the transition it turns out that the strength of such fluctuations governs the location of the first-order transition and affect the development of the accompanying hump in the specific heat of MnSi. Our data indicate that (i) there is a crossover from a second-order to a first-order transition with increasing D/J and (ii) the observed broad hump originates from highly developed helical fluctuations, which manifest themselves in the transient multiple spiral state.